The specifications of the MPEG1 standard state that the treated pictures are necessarily in a sequential form. If the original pictures are television pictures, i.e. pictures having two interlaced fields, these two fields considered for MPEG encoding as information components taken at the same instant correspond, however, to a lesser extent to data which in reality are spaced apart by inter-field intervals of 20 ms. In picture sequences in which the motion is very rapid, such a significant inter-field motion contributes to picture faults, such as comb effects which are due to the appearance of parasitic frequencies in the blocks.
In accordance with the MPEG1 standard each picture is cut up into macroblocks of 16.times.16 pixels comprising four blocks of 8.times.8 pixels for the luminance and two blocks of 8.times.8 pixels for the chrominance. FIG. 1 shows in such a macroblock a vertical structure whose alignment is satisfactorily maintained in the absence of motion, even if the lines of this macroblock alternately correspond to the one or the other of the two interlaced fields which are 20 ms apart. FIG. 2 shows the same vertical structure in the case where the inter-field motion is significant and where the comb effect appears due to time-shifting of the information components of the odd lines and those of the even lines of the macroblock.
U.S. Pat. No. 5,091,782 describes a device for encoding digital video signals in which the two interlaced fields are treated in two distinct manners before encoding. On the one hand, the interlaced fields are separated and then subdivided into blocks which are subsequently each submitted to an orthogonal transform followed by a quantization, and on the other hand the blocks thus obtained and being in spatial conformity in the two fields are re-interlaced so as to be submitted to an orthogonal transform and a quantization. Error computations with respect to the signals before quantization are respectively effected on the two distinct sequences of signals thus quantized so that, in accordance with the motion in the original signals, the definitive selection of one of said two quantized signal sequences to be encoded can be made, viz. the sequence with which the smallest error is associated.